This invention relates to a backplane wiring scheme as used in hub in a packet data communications system, and more particularly to a backplane wiring scheme which allows line cards to be connected as either a bus or a ring configuration.
When a number of line cards are connected to a backplane wiring arrangement in a hub for a data communications system, where each line card is a network segment, a station or a bridge to other stations or segments, the connection is usually in either a ring or a bus topology.
A bus topology uses a collection of parallel wires running across the backplane. Each line card connects to the bus and uses one or more of the bus wires to communicate with the other line cards. The bus is often divided into channels, each channel containing some fixed number of bus wires. The bus wires in a given channel are all used in the same manner, whereas different channels can be used independently. A channel can be used as a broadcast bus, with some specified arbitration scheme, to provide a dynamic communication capability between two or more line cards; or it can be used in a fixed manner with the channel forming a dedicated communication link between two or more cards. the broadcast bus scheme is typically used to support connections amongst Ethernet repeaters in smart hubs; the fixed channel scheme is often used to support ring topologies as found in FDDI wiring concentrators. Bus topologies often require the use of bus-terminating resistors placed at each end of the bus to improve the electrical performance. Without these resistors, communication bandwidth is compromised.
The ring topology is used exclusively to support interconnection amongst line cards that are wired together in a closed one-way ring. The backplane connects the output of one line card to the input of the next line card. The disadvantage of this topology is (as with Christmas tree lights) if one line card dies or is removed, the whole ring dies. Palliative schemes to overcome this problem have been suggested, but none match the richness of interconnect offered by bus topologies.
When a collection of channels are wired in a ring topology, the line cards must be able to transmit onto and receive from any of the other channels (though the selection will be fixed once the ring topology is established). This expense can be avoided by providing a dedicated ring topology to the backplane, as will be described.